Absorption spectroscopy in the ultraviolet (UV) wavelength range is a widely used method for analysis of organic materials which absorb UV radiation. Due to the high absorptivities (i.e., large extinction coefficients) of UV absorbing compounds, absorption measurements customarily are performed on samples diluted with UV transparent solvents. Dilution factors of 100 to 10,000 commonly are employed in conjunction with absorption cells having pathlengths of 1 mm to 10 cm. Absorption measurements of undiluted samples would require the use of cells with pathlengths on the order of 1 micrometer. The necessity of either diluting the sample or employing absorption cells of inconveniently short pathlengths has severely limited the application of UV absorption spectroscopy for in-situ analyses in chemical processes.